Since its introduction in the late 1970s in a report by the Canadian Task Force on Periodic Health Examination (1979), the hierarchy of evidence has evolved from ranking original studies to incorporating pre-appraised/filtered resources, such as the 6S model by Haynes and colleagues (Dicenso, Bayley, Haynes, 2009) to the Oxford Centre for Evidence-Based Medicine's Level of Evidence Table (OCEMB Levels of Evidence Working Group, 2011).
The traditional hierarchy of evidence that supported therapeutic interventions shows the inverse relationship between quantity and quality/relevance of primary studies and other information sources. At the top of the pyramid are the secondary research papers, systematic reviews/meta-analyses, generally considered the highest level of evidence. In systematic reviews/meta-analyses, exhaustive literature searches are performed to retrieve the primary studies on the specific topic and then the studies are critically appraised according to rigorous criteria. (Greenhalgh, 2019)
When you do not find the answer in a systematic review/meta-analysis, you may need to go down the pyramid and review other resources. It is important to remember when evaluating the merits of an individual study, it involves more than checking its study design versus an evidence hierarchy/pyramid. All information is not necessarily equivalent. For example, a methodically flawed randomized controlled trial should not be placed above a well-designed cohort study.(Greenhalgh, 2019)
Issues of diagnosis or prognosis require different hierarchies. For studies of the accuracy of diagnostic tests, the top of the hierarchy includes studies that enrolled patients about whom clinicians had diagnostic uncertainty and that undertook a blind comparison between the candidate test and a criterion standard (see Chapter 18, Diagnostic Tests, and Chapter 20, Prognosis, in User's Guides to the Medical Literature: A Manual for Evidence-Based Clinical Practice, 3rd ed.). For prognosis, prospective observational studies accurately documenting exposures and outcomes and following up all patients during relevant periods would sit atop the hierarchy. (Guyatt, 2015).